Warfarin Institute of America
DEDICATED TO YOUR HEALTH SINCE 2000
Katy M. Ryer, PharmD. Candidate
University of Colorado at Denver Health Sciences Center
After heart valve surgery, patients must take an anticoagulant, usually warfarin, to prevent blood clots from forming in the heart1. Once a patient begins warfarin therapy, he/she will be required to get their blood tested regularly to determine their International Normalized Ratio, or INR. The American Heart Association and the American College of Cardiology have established guidelines stating that patients who have undergone a prosthetic heart valve replacement should maintain an INR between 2.5 and 3.5 in most cases1.
Many patients who take warfarin will have their dose adjusted at one point or another. Patients who have undergone heart valve replacement may have more frequent adjustments in their dose following surgery. Several studies have shown that patients have increased sensitivity to warfarin following heart valve replacements. This increased sensitivity means that patients will require a lower dose of warfarin following their heart valve surgery. The initial sensitivity to the warfarin following surgery is short lived and patients will usually require an increase in their warfarin dose several weeks after surgery to maintain their goal INR.
During heart valve surgery patients undergo cardiopulmonary bypass, which means a machine is responsible for pumping their blood during surgery. During this process, proteins and certain clotting factors are removed from the blood. Normally, warfarin binds very strongly to proteins in the blood and it works to thin the blood by inhibiting clotting factors. The decreased amount of proteins and clotting factors in the blood following heart valve surgery is one of the reasons patients are more sensitive to warfarin after their surgery2,3.
Mesbah Rahman and colleagues conducted a study evaluating warfarin sensitivity in 111 patients following heart valve surgery. Results of the study showed 25% of patients experienced an elevated INR greater then 4.0 during their hospitalization after surgery while only 10% of patients experienced an elevated INR greater then 4.0 during the follow up period. The follow up period was defined as four consecutive INR values over a period of 4-12 weeks after surgery. This data suggests that in the weeks following heart valve surgery, the initial sensitivity to warfarin decreases2.
There are several other factors that can contribute to a high INR following heart valve surgery. Following surgery patients are less active and therefore have decreased circulation. Warfarin is broken down in the body by the liver and decreased circulation of warfarin through the liver increases the amount of time warfarin is in the body which can result in an elevated INR4. Also after surgery, patients are not eating as well as they normally do and often time vegetables are cut from the diet. Since vegetables are high in vitamin K, abruptly cutting them from the diet can also result in an elevated INR4.
A patient’s warfarin dose may fluctuate following heart valve surgery. A variety of factors such as the cardiopulmonary bypass during surgery, inactivity, and changes in diet may all result in an elevated INR initially following surgery. Once patients return to health and are eating a more regular diet, they may require more warfarin to maintain their goal INR. It is important to realize that close monitoring of the INR is imperative following heart valve surgery.
References:
© 2006 Katy Ryer
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Last updated May 9, 2006